Fig.
8The essential features of our Foucault pendulum, which is based on
Crane's design, are shown in Fig. 8. Of particular note is the Charron
ring used to limit the elipticity and the pair of magnets A and B with
magnet B adjustable to alter the force law and reduce the precession resulting
from any residual elipticity. Finally there is the magnetic drive coil
which is driven in phase with the pendulum and creates a push-pull force
on magnet A to keep the pendulum going against friction. The drive current
is controlled by electronics under the base which are in turn triggered
by an induction current in the sense coil every time the pendulum magnet
A sweeps by. An LED under the base support flashes on whenever the drive
is in its push mode.